Foams based on thermoplastic polyurethanes

ABSTRACT

Expandable thermoplastic polyurethane comprising blowing agent, wherein the Shore hardness of the thermoplastic polyurethane is from A 44 to A 84.

The invention relates to expandable thermoplastic polyurethane,preferably in bead form, comprising blowing agent, where the Shorehardness of the thermoplastic polyurethane is from A 44 to A 84,preferably from A 62 to A 82, particularly preferably from A 62 to A 80.The Shore hardness of the TPU here is measured on the compact, i.e.unexpanded, TPU. The invention moreover relates to processes forproduction of expandable thermoplastic polyurethane, preferably in beadform, comprising blowing agent. The invention also relates to processesfor production of expanded thermoplastic polyurethane, and to processesfor production of foam based on thermoplastic polyurethane, and to foamsor expanded thermoplastic polyurethanes thus obtainable.

Foams, and this particularly applies to moldable foams, have been knownfor a long time and are widely described in the literature, e.g. inUllmann's “Encyklopädie der technischen Chemie” [Encyclopedia ofIndustrial Chemistry], 4th edition, Volume 20, pp. 416 et seq.

DE 4015714 A1 mentions glass fiber-reinforced TPU foams which areproduced in an injection-molding machine. The examples state densitiesof 800 g/L and greater. These are foamed TPU sheets, not moldable foams.

Moldable foams based on thermoplastic polyurethane, also termed TPU inthis specification, have been disclosed in WO 94/20568. A disadvantageof the TPU foams described in WO 94/20568 is the high energy consumptionduring production and processing. A steam pressure of from 4.5 bar to 7bar is used, i.e. a temperature of from 145° C. to 165° C.

WO 94/20568 also describes expanded, i.e. foamed, TPU beads which can beprocessed to give moldings. These TPU foam beads are produced attemperatures of 150° C. and higher and in the examples have a bulkdensity of from 55 to 180 g/L, with resultant disadvantage in transportand storage of these beads due to the increased space required.

The object of the present invention therefore consists in developing amoldable TPU foam which can be produced at low temperatures andsimultaneously has good performance in relation to elasticity and totemperature variation. A further object was to develop expandable TPUbeads and expanded TPU foam beads, and processes for their production,these being beads which can be produced and processed at lowtemperatures.

These objects have been achieved via expandable thermoplasticpolyurethane, preferably in bead form, comprising blowing agent, wherethe Shore hardness of the thermoplastic polyurethane is from A 44 to A84, preferably from A 62 to A 82, particularly preferably from A 62 to A80. The Shore hardness of the TPU here is measured on the compact, i.e.unexpanded, TPU.

The advantage of the present invention is that it uses TPU with lowerhardness, lower melting point, and better flowability. The result isthat temperatures and pressures can be kept lower during the productionof the expanded TPU beads. Specifically when steam is used, it isadvantageous to be able to operate at lower temperatures. Furthermore,the softness makes adhesive-bonding of the foam beads more effective.

According to the invention, preferred TPUs are those in which themelting range measured by DSC with a heating rate of 20 K/min startsbelow 130° C., more preferably below 120° C., and the thermoplasticpolyurethane has at most a melt flow rate (MFR) of 250 g/10 min,particularly preferably smaller than 200 g/10 min at 190° C. with anapplied weight of 21.6 kg to DIN EN ISO 1133.

Another advantage of the inventive thermoplastic polyurethanes consistsin their better feel.

The inventive TPUs are preferably based on polyether alcohol,particularly preferably polyetherdiol. Polytetrahydrofuran canparticularly preferably be used here. It is particularly preferable thatthe TPU is based on polytetrahydrofuran whose molar mass is from 600g/mol to 2500 g/mol. The polyether alcohols can be used eitherindividually or else in a mixture with one another.

As an alternative, good results were achieved with TPU based onpolyester alcohol, preferably polyesterdiol, particularly preferablyderived from adipic acid and 1,4-butanediol, with a molar mass of from600 g/mol to 900 g/mol.

Thermoplastic polyurethanes and processes for their production are wellknown. By way of example, TPUs can be produced via reaction of (a)isocyanates with (b) compounds reactive toward isocyanates and having amolar mass of from 500 to 10000 and, if appropriate, (c) chain extendershaving a molar mass of from 50 to 499, if appropriate in the presence of(d) catalysts and/or of (e) conventional auxiliaries and/or conventionaladditives.

The starting components and processes for production of the preferredpolyurethanes will be described by way of example below. The components(a), (b), and also, if appropriate, (c), (d) and/or (e) usually used inproduction of the polyurethanes will be described by way of examplebelow:

-   -   a) Organic isocyanates (a) which may be used are well-known        aliphatic, cycloaliphatic, araliphatic, and/or aromatic        isocyanates, preferably diisocyanates, for example tri-, tetra-,        penta-, hexa-, hepta- and/or octamethylene diisocyanate,        2-methylpentamethylene 1,5-diisocyanate, 2-ethylbutylene        1,4-diisocyanate, pentamethylene 1,5-diisocyanate, butylene        1,4-diisocyanate,        1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane        (isophorone diisocyanate, IPDI), 1,4-and/or        1,3-bis(isocyanatomethyl)cyclohexane (HXDI), cyclohexane        1,4-diisocyanate, 1-methylcyclohexane 2,4- and/or        2,6-diisocyanate, and/or dicyclohexylmethane 4,4′-, 2,4′- and        2,2′-diisocyanate, diphenylmethane 2,2′-, 2,4′- and/or        4,4′-diisocyanate (MDI), naphthylene 1,5-diisocyanate (NDI),        tolylene 2,4- and/or 2,6-diisocyanate (TDI), diphenylmethane        diisocyanate, 3,3′-dimethylbiphenyl diisocyanate,        1,2-diphenylethane diisocyanate, and/or phenylene diisocyanate.    -   b) Compounds (b) which may be used and are reactive toward        isocyanates are the well-known compounds reactive toward        isocyanates, for example polyesterols, polyetherols, and/or        polycarbonatediols, these usually also being combined under the        term “polyols”, having molar masses of from 500 to 8000,        preferably from 600 to 6000, in particular from 800 to 4000, and        preferably having an average functionality of from 1.8 to 2.3,        preferably from 1.9 to 2.2, in particular 2.    -   c) Chain extenders (c) that may be used-comprise well-known        aliphatic, araliphatic, aromatic and/or cycloaliphatic compounds        having a molar mass of from 50 to 499, preferably difunctional        compounds, such as diamines and/or alkanediols having from 2 to        10 carbon atoms in the alkylene radical, in particular        1,4-butanediol, 1,6-hexanediol, and/or di-, tri-, tetra-,        penta-, hexa-, hepta-, octa-, nona- and/or decaalkylene glycols        having from 3 to 8 carbon atoms, and preferably corresponding        oligo- and/or polypropylene glycols, and use may also be made of        a mixture of the chain extenders.    -   d) Suitable catalysts which in particular accelerate the        reaction between the NCO groups of the diisocyanates (a) and the        hydroxy groups of the structural components (b) and (c) are the        conventional tertiary amines known from the prior art, a g.        triethylamine, dimethylcyclohexylamine, N-methylmorpholine,        N,N′-dimethylpiperazine, 2-(dimethylaminoethoxy)ethanol,        diazabicyclo-[2.2.2]octane and the like, and also in particular        organometallic compounds, such as titanic esters, iron        compounds, e.g. ferric acetylacetonate, tin compounds, e.g.        stannous diacetate, stannous dioctoate, stannous dilaurate, or        the dialkyltin salts of aliphatic carboxylic acids, e.g.        dibutyltin diacetate, dibutyltin dilaurate, or the like. The        amounts usually used of the catalysts are from 0.0001 to 0.1        part by weight per 100 parts by weight of polyhydroxy compound        (b).    -   e) Alongside catalysts (d), conventional auxiliaries and/or        additives (e) may also be added to the structural components (a)        to (c). By way of example, mention may be made of blowing        agents, surface-active substances, fillers, flame retardants,        nucleating agents, antioxidants, lubricants and mold-release        agents, dyes and pigments, further stabilizers if appropriate in        addition to the inventive stabilizer mixture, e.g. with respect        to hydrolysis, light, heat or discoloration, inorganic and/or        organic fillers, reinforcing agents, and plasticizers. In one        preferred embodiment, component (e) also includes hydrolysis        stabilizers, such as polymeric and low-molecular-weight        carbodiimides. In another embodiment, the TPU can comprise a        phosphorus compound. In one preferred embodiment, phosphorus        compounds used are organophosphorus compounds of trivalent        phosphorus, examples being phosphites and phosphonites. Examples        of suitable phosphorus compounds are triphenyl phosphate,        diphenyl alkyl phosphate, phenyl dialkyl phosphite,        tris(nonylphenyl) phosphite, trilauryl phosphite, trioctadecyl        phosphite, distearyl pentaerythritol diphosphite,        tris(2,4-di-tert-butylphenyl) phosphite, diisodecyl        pentaerythritol diphosphite, di(2,4-di-tert-butylphenyl)        pentaerythritol diphosphite, tristearyl sorbitol triphosphite,        tetrakis(2,4-di-tert-butylphenyl) 4,4′-diphenylenediphosphonite,        trisisodecyl phosphite, diisodecyl phenyl phosphite, and        diphenyl isodecyl phosphite, or a mixture thereof.

The phosphorus compounds are particularly suitable when they aredifficult to hydrolyze, since the hydrolysis of a phosphorus compound togive the corresponding acid can lead to degradation of the polyurethane,in particular of the polyester urethane. Accordingly, the phosphoruscompounds particularly suitable for polyester urethanes are those whichare particularly difficult to hydrolyze. Examples of these phosphoruscompounds are dipolypropylene glycol phenyl phosphite, triisodecylphosphite, triphenyl monodecyl phosphite, trisisononyl phosphite,tris(2,4-di-tert-butylphenyl) phosphite,tetrakis(2,4-di-tert-butylphenyl)-4,4′-biphenylylene diphosphonite, anddi(2,4-di-tert-butylphenyl)-pentaerythritol diphosphite, or a mixturethereof.

Fillers that can be used are organic and inorganic powders or fibrousmaterials, or else a mixture thereof. Examples of organic fillers thatcan be used are wood flour, starch, flax fibers, hemp fibers, ramiefibers, jute fibers, sisal fibers, cotton fibers, cellulose fibers, oraramid fibers. Examples of inorganic fillers that can be used aresilicates, barite, glass beads, zeolite, metals or metal oxides. It ispreferable to use pulverulent inorganic substances, such as talc, chalk,kaolin, (Al₂(Si₂O₅)(OH)₄), aluminum hydroxide, magnesium hydroxide,aluminum nitrite, aluminum silicate, barium sulfate, calcium carbonate,calcium sulfate, silica, powdered quartz, Aerosil, alumina, mica, orwollastonite, or inorganic substances in the form of beads or fibers,e.g. iron powder, glass beads, glass fibers, or carbon fibers. Theaverage particle diameters or, in the case of fillers in the form offibers, the length should be in the region of the cell size or smaller.Preference is given to an average particle diameter in the range from0.1 to 100 μm, preferably in the range from 1 to 50 μm. Preference isgiven to expandable, thermoplastic polyurethanes comprising blowingagent and comprising from 5 to 80% by weight of organic and/or inorganicfillers, based on the total weight of the thermoplastic polyurethanecomprising blowing agent. Further preference is given to expandedthermoplastic polyurethanes which comprise from 5 to 80% by weight oforganic and/or inorganic fillers, based on the total weight of thethermoplastic polyurethane.

Besides the components a) and b) mentioned, and if appropriate, c), d)and e), it is also possible to use chain regulators, usually with molarmass of from 31 to 499. These chain regulators are compounds which haveonly one functional group reactive toward isocyanates, examples beingmonohydric alcohols, monobasic amines, and/or monohydric polyols. Thesechain regulators can give precise control of flow behavior, inparticular in the case of TPUs. The amount of chain regulators which maygenerally be used is from 0 to 5 parts by weight, preferably from 0.1 to1 part by weight, based on 100 parts by weight of component b), and thechain regulators are defined as part of component (c).

All of the molar masses mentioned in this specification have the unit[g/mol].

To adjust the hardness of the TPUs, the molar ratios of the structuralcomponents (b) and (c) may be varied relatively widely. Successful molarratios of component (b) to the entire amount of chain extenders (c) tobe used are from 10:1 to 1:10, in particular from 1:1 to 1:4, and thehardness of the TPUs here rises as content of (c) increases.

It is preferable that chain extenders (c) are also used for productionof the TPUs.

The reaction can take place at conventional indices, preferably with anindex of from 60 to 120, particularly preferably at an index of from 80to 110. The index is defined via the ratio of the total number ofisocyanate groups used during the reaction in component (a) to thenumber of groups reactive toward isocyanates, i.e. to the activehydrogen atoms, in components (b) and (c). If the index is 100, there isone active hydrogen atom, i.e. one function reactive toward isocyanates,in components (b) and (c) for each isocyanate group in component (a). Ifindices are above 100, there are more isocyanate groups than OH groupspresent.

The TPUs can be produced by the known processes continuously, forexample using reactive extruders, or the belt process, by the one-shotmethod or the prepolymer method, or batchwise by the known prepolymerprocess. The components (a), (b) and, if appropriate, (c), (d), and/or(e) reacting in these processes can be mixed with one another insuccession or simultaneously, whereupon the reaction immediately begins.

In the extruder process, structural components (a), (b), and, ifappropriate, (c), (d), and/or (e) are introduced individually or in theform of a mixture into the extruder, e.g. at temperatures of from 100 to280° C., preferably from 140 to 250° C., and reacted, and the resultantTPU is extruded, cooled, and pelletized. It can, if appropriate, beadvisable to heat-condition the resultant TPU prior to furtherprocessing at from 80 to 120° C., preferably from 100 to 110° C., for aperiod of from 1 to 24 hours.

According to the invention, the inventive TPUs described at the outsetare used for production of the expandable thermoplastic polyurethanes,preferably in bead form, comprising blowing agent, for production ofexpanded thermoplastic polyurethane, and for production of foam based onthermoplastic polyurethane. The production of these materials from theinventive TPUs is described below.

In principle the inventive expanded TPU beads can be produced viasuspension or extrusion processes directly or indirectly by way ofexpandable TPU beads and foaming in a pressure prefoamer with steam orhot air.

In the suspension process, the TPU in the form of pellets is heated withwater, with a suspending agent, and with the blowing agent in a closedreactor to above the softening point of the pellets. The polymer beadsare thereby impregnated by the blowing agent. It is then possible eitherto cool the hot suspension, whereupon the particles solidify withinclusion of the blowing agent, and to depressurize the reactor. The(expandable) beads comprising blowing agent and obtained in this way arefoamed via heating to give the expanded beads. As an alternative, it ispossible to depressurize the hot suspension suddenly, without cooling(explosion-expansion process), whereupon the softened beads comprisingblowing agent immediately foam to give the expanded beads, see, forexample, WO 94/20568.

In the extrusion process, the TPU is mixed, with melting, in an extruderwith a blowing agent which is introduced into the extruder. Either themixture comprising blowing agent is extruded and pelletized underconditions of pressure and temperature such that the TPU pellets do notfoam (expand), an example of a method being used for this purpose beingunderwater pelletization, which is operated with a water pressure ofmore than 2 bar. This gives expandable beads comprising blowing agent,which are then foamed via subsequent heating to give the expanded beads.Or the mixture can also be extruded and pelletized at atmosphericpressure. In this process, the melt extrudate foams and the productobtained via pelletization is the expanded beads.

The TPU can be used in the form of commercially available pellets,powder, granules, or in any other form. It is advantageous to usepellets. An example of a suitable form is what are known as minipelletswhose preferred average diameter is from 0.2 to 10 mm, in particularfrom 0.5 to 5 mm. These mostly cylindrical or round minipellets areproduced via extrusion of the TPU and, if appropriate, of otheradditives, discharged from the extruder, and if appropriate cooling, andpelletization. In the case of cylindrical minipellets, the length ispreferably from 0.2 to 10 mm, in particular from 0.5 to 5 mm. Thepellets can also have a lamellar shape. The average diameter of thethermoplastic polyurethane comprising blowing agent is preferably from0.2 to 10 mm.

The expandable TPU beads of the invention can be produced by thesuspension process or by the extrusion process.

As a function of the process used, the preferred blowing agents can varyif appropriate. In the case of the suspension process, the blowing agentused preferably comprises organic liquids or inorganic gases, or amixture thereof. Liquids that can be used comprise halogenatedhydrocarbons, but preference is given to saturated, aliphatichydrocarbons, in particular those having from 3 to 8 carbon atoms.Suitable inorganic gases are nitrogen, air, ammonia, or carbon dioxide.

In production via an extrusion process, the blowing agent usedpreferably comprises volatile organic compounds whose boiling point atatmospheric pressure of 1013 mbar is from −25 to 150° C., in particularfrom −10 to 125° C. Hydrocarbons (preferably halogen-free) have goodsuitability, in particular C₄₋₁₀-alkanes, for example the isomers ofbutane, of pentane, of hexane, of heptane, and of octane, particularlypreferably sec-pentane. Other suitable blowing agents are bulkiercompounds, examples being alcohols, ketones, esters, ethers, and organiccarbonates.

It is also possible to use halogenated hydrocarbons, but the blowingagent is preferably halogen-free. Very small proportions ofhalogen-containing blowing agents in the blowing agent mixture arehowever not to be excluded. It is, of course, also possible to usemixtures of the blowing agents mentioned.

The amount of blowing agent is preferably from 0.1 to 40 parts byweight, in particular from 0.5 to 35 parts by weight, and particularlypreferably from 1 to 30 parts by weight, based on 100 parts by weight ofTPU used.

In the suspension process, operations are generally carried outbatchwise in an impregnator, e.g. in a stirred-tank reactor. The TPU isfed, e.g. in the form of minipellets, into the reactor, as also is wateror another suspension medium, and the blowing agent and, if appropriate,a suspending agent. Water-insoluble inorganic stabilizers are suitableas suspending agent, examples being tricalcium phosphate, magnesiumpyrophosphate, and metal carbonates; and also polyvinyl alcohol andsurfactants, such as sodium dodecylarylsulfonate. The amounts usuallyused of these are from 0.05 to 10% by weight, based on the TPU.

The reactor is then sealed, and the reactor contents are heated to animpregnation temperature which is usually at least 100° C. The blowingagent here can be added prior to, during, or after heating of thereactor contents. The impregnation temperature should be in the vicinityof the softening point of the TPU. Impregnation temperatures of from 100to 150° C., in particular from 110 to 145° C., are preferred.

As a function of the amount and nature of the blowing agent, and also ofthe temperature, a pressure (impregnation pressure) becomes establishedin the sealed reactor and is generally from 2 to 100 bar (absolute). Thepressure can, if necessary, be regulated via a pressure-control valve orvia introduction of further blowing agent under pressure. At theelevated temperature and superatmospheric pressure provided by theimpregnation conditions, blowing agent diffuses into the polymerpellets. The impregnation time is generally from 0.5 to 10 hours.

In one embodiment of the suspension process, cooling of the heatedsuspension, usually to below 100° C., takes place after the impregnationprocess, the result being re-solidification of the TPU and inclusion ofthe blowing agent. The material is then depressurized. The product isexpandable TPU beads which finally are conventionally isolated from thesuspension. Adherent water is generally removed via drying, e.g. in apneumatic dryer. Subsequently or previously, if necessary, adherentsuspending agent can be removed by treating the beads with a suitablereagent. By way of example, treatment with an acid, such as nitric acid,hydrochloric acid, or sulfuric acid, can be used in order to removeacid-soluble suspending agents, e.g. metal carbonates or tricalciumphosphate.

In the extrusion process, it is preferable that the TPU, the blowingagent and, if appropriate, additives are introduced together (in theform of a mixture) or separately from one another at one or variouslocations of the extruder. The possibility, but not a requirement, hereis to prepare a mixture in advance from the solid components. By way ofexample, it is possible to begin by mixing TPU and, if appropriate,additives, and to introduce the mixture into the extruder, and thenintroduce the blowing agent into the extruder, so that the extrudermixes the blowing agent into to polymer melt. It is also possible tointroduce a mixture of blowing agent and additives into the extruder,i.e. to premix the additives with the blowing agent.

In the extruder, the starting materials mentioned are mixed, withmelting of the TPU.

Any of the conventional screw-based machines can be used as extruder, inparticular single-screw and twin-screw extruders (e.g. Werner &Pfleiderer ZSK machines), co-kneaders, Kombiplast machines, MPC kneadingmixers, FCM mixers, KEX kneading screw extruders, and shear-rollextruders, as described by way of example in Saechtling (ed.),Kunststoff-Taschenbuch [Plastics handbook], 27th edition, Hanser-VerlagMunich 1998, chapter 3.2.1 and 3.2.4. The extruder is usually operatedat a temperature at which the TPU is present in the form of a melt, forexample at from 150 to 250° C., in particular from 180 to 210° C.

The rotation, length, diameter, and design of the extruder screw(s),amounts introduced, and extruder throughput, are selected in a knownmanner in such a way as to give uniform distribution of the additives inthe extruded TPU.

In one embodiment of the extrusion process, expandable beads areproduced. To prevent premature foaming of the melt comprising blowingagent on discharge from the extruder, the melt extrudate is dischargedfrom the extruder and pelletized under conditions of temperature andpressure such that practically no foaming (expansion) occurs. Theseconditions can vary as a function of the type and amount of thepolymers, of the additives, and in particular of the blowing agent. Theideal conditions can easily be determined via preliminary experiments.

One industrially advantageous method is underwater pelletization in awaterbath whose temperature is below 100° C. and which is subject to apressure of at least 2 bar (absolute). Excessively low temperature hasto be avoided, because otherwise the melt hardens on the die plate, andexcessively high temperature has to be avoided since otherwise the meltexpands. As the boiling point of the blowing agent increases and theamount of the blowing agent becomes smaller, the permissible watertemperature becomes higher and the permissible water pressure becomeslower. In the case of the particularly preferred blowing agentsec-pentane, the ideal waterbath temperature is from 30 to 60° C. andthe ideal water pressure is from 8 to 12 bar (absolute). It is alsopossible to use other suitable coolants instead of water. It is alsopossible to use water-cooled die-face pelletization. In this process,encapsulation of the cutting chamber is such as to permit operation ofthe pelletizing apparatus under pressure.

The product is expandable TPU beads, which are then isolated from thewater and, if appropriate, dried. They are then foamed as described at alater stage below, to give expanded TPU beads.

A preferred process for production of expandable TPU beads comprisingblowing agent comprises the following stages:

-   -   i) melting of TPU, if appropriate with additives, and extrusion        to give pellets whose average diameter is from 0.2 to 10 mm,    -   ii) impregnation of the pellets with from 0.1 to 40% by weight,        based on the total weight of the pellets, of a volatile blowing        agent in aqueous suspension under pressure, preferably at a        pressure of from 5 to 100 bar, at temperatures in the range from        100 to 150° C.,    -   iii) cooling of the suspension to from 20 to 95° C.,    -   iv) then depressurizing.

Via cooling, the blowing agent becomes included within the polymer, andthe product does not foam. If the tank is depressurized directly at hightemperatures in step ii), the blowing agent escapes, and the polymer,which is soft at these temperatures, expands.

Another preferred process for production of expandable TPU beadscomprising blowing agent comprises the following stages:)

-   -   i) melting of TPU together with from 0.1 to 40% by weight, based        on the total weight of the pellets, of a volatile blowing agent        and, if appropriate, with additives, in an extruder,    -   ii) discharge of the melt from the extruder and underwater        pelletization of the melt extrudate at pressures of from 2 bar        to 20 bar and temperatures of from 5° C. to 95° C.

This process uses pelletization under water against superatmosphericpressure to avoid escape of the blowing agent and foaming of thepolymer.

The invention therefore also provides, and this is particularlypreferred, a process for production of expandable thermoplasticpolyurethane, preferably in bead form, comprising blowing agent, where athermoplastic polyurethane whose Shore hardness is from A 44 to A 84,preferably from A 62 to A 80, is extruded, if appropriate together withadditives, to give pellets whose average diameter is from 0.2 to 10 mm,the pellets are impregnated with from 0.1 to 40% by weight, based on thetotal weight of the pellets, of a preferably volatile blowing agent inaqueous suspension under pressure, preferably at a pressure of from 5 to100 bar, at temperatures in the range from 100 to 150° C., thesuspension comprising the thermoplastic polyurethanes comprising blowingagent is cooled to from 20 to 95° C., and then the thermoplasticpolyurethanes comprising blowing agent are depressurized.

The invention therefore also provides, and this is particularlypreferred, a process for production of expandable thermoplasticpolyurethane, preferably in bead form, comprising blowing agent, where athermoplastic polyurethane whose Shore hardness is from A 44 to A 84,preferably from A 62 to A 80, is melted together with from 0.1 to 40% byweight, based on the total weight of the pellets, of a preferablyvolatile blowing agent and, if appropriate, with additives, in anextruder, and the melt is pelletized under water at pressures of from 2bar to 20 bar and temperatures of from 5° C. to 95° C.

To the extent that expandable beads are obtained, these can be foamed ina known manner, whereupon the inventive expanded TPU beads are produced.The foaming generally takes place via heating of the expandable beads inconventional foaming apparatuses, e.g. with hot air or superheated steamin what is known as a pressure prefoamer, for example of the typeusually used for processing of expandable polystyrene (EPS). It ispreferable to foam the beads at a temperature at which they soften(softening range), particularly preferably at temperatures of from 100to 140° C.

The present invention therefore also provides a process for productionof foams based on thermoplastic polyurethane, where the inventiveexpandable thermoplastic polyurethane, preferably in bead form,comprising blowing agent is foamed at a temperature of from 100° C. to140° C. The present invention also provides foams thus obtainable andbased on thermoplastic polyurethane.

If steam is used for foaming, the steam pressure is usually, as afunction of the nature and amount of TPU and blowing agent, and of thedesired density of the foam to be produced, from 1 to 4 bar (absolute),preferably from 1.5 to 3.5 bar (absolute). As the pressures increasehere the densities of the foamed TPU product become smaller, i.e. steampressure can be used to set the desired density. The foaming time isusually from 1 to 300 sec, preferably from 1 to 30 sec. Foaming isfollowed by depressurization and cooling. The expansion factor duringfoaming is preferably from 2 to 50.

In one embodiment of the suspension process for production of theexpanded TPU beads, the heated suspension is not cooled, butdepressurized suddenly while hot, without cooling. Duringdepressurization, the blowing agent which has previously diffused intothe TPU beads expands “explosively” and foams the softened beads.Expanded TPU beads are obtained.

The suspension is usually depressurized via a die, a valve, or anothersuitable apparatus. The suspension can be directly depressurized toatmospheric pressure, such as 1013 mbar. However, it is preferable todepressurize in an intermediate container whose pressure is sufficientfor foaming of the TPU beads but can be above atmospheric pressure. Asuitable method depressurizes to a pressure of, for example, from 0.5 to5 bar (absolute), in particular from 1 to 3 bar (absolute). During thedepressurization process, the impregnation pressure in the impregnationcontainer can be kept constant, by introducing further blowing agentunder pressure. The method generally used comprises cooling of thesuspension after depressurization, isolation of the expanded TPU beadsconventionally from the suspension, and, before that or after that, ifappropriate, removal of adherent suspending agent, as described above,and finally washing and drying of the beads.

In one embodiment of the extrusion process for production of theexpanded TPU beads, the melt comprising blowing agent is discharged fromthe extruder and pelletized without underwater pelletization,water-cooled die-face pelletization or other precautions which inhibitfoaming. By way of example, extrusion can take place directly into theatmosphere. The melt extrudate discharged from the extruder foams duringthis process, and expanded TPU beads are obtained via pelletization ofthe foamed extrudate.

A preferred process for production of expanded TPU beads comprises thefollowing stages

-   -   i) melting of TPU, if appropriate with additives, and extrusion        to give minipellets whose average diameter is from 0.2 to 10 mm,    -   ii) impregnation of the minipellets with from 0.1 to 40% by        weight, based on the total weight of the pellets, of a volatile        blowing agent in aqueous suspension under pressure, preferably        at a pressure of from 5 to 100 bar, at temperatures in the range        from 100 to 150° C., and    -   iii) then depressurization.

Another preferred process for expansion of expanded TPU beads comprisesthe following stages:

-   -   i) melting of TPU together with from 0.1 to 40% by weight, based        on the total weight of the pellets, of a volatile blowing agent        and, if appropriate, with additives; in an extruder,    -   ii) discharge of the melt from the extruder and pelletization of        the melt extrudate without apparatuses which inhibit foaming.

The invention also provides, and this is particularly preferred, aprocess for production of expanded thermoplastic polyurethane, where athermoplastic polyurethane whose Shore hardness is from A 44 to A 84,preferably from A 62 to A 80, is extruded, if appropriate together withadditives, to give pellets whose average diameter is from 0.2 to 10 mm,the pellets are impregnated with from 0.1 to 40% by weight, based on thetotal weight of the pellets, of a preferably volatile blowing agent,preferably in aqueous suspension under pressure, preferably at apressure of from 5 to 100 bar, at temperatures in the range from 100 to150° C., and then are depressurized.

The invention also provides, and this is particularly preferred, aprocess for production of expanded thermoplastic polyurethane, where athermoplastic polyurethane whose Shore hardness is from A 44 to A 84,preferably from A 62 to A 80, is melted together with from 0.1 to 40% byweight, based on the total weight of the pellets, of a preferablyvolatile blowing agent, if appropriate with additives, in an extruder,and the melt is pelletized without apparatuses which inhibit foaming.

The present invention also provides expanded thermoplastic polyurethanesobtainable via these process.

The TPU beads can be provided, prior to and/or after the foamingprocess, with an antiblocking agent. Examples of suitable antiblockingagents are talc, metal compounds, such as tricalcium phosphate, calciumcarbonate, silicas, in particular fumed silicas, such as Aerosil® fromDegussa, salts of long-chain (e.g. C₁₀₋₂₂) carboxylic acids, for examplestearic salts, such as calcium stearate, esters of long-chain carboxylicacids, e.g. glycerol esters, such as the glycerol stearates, andsilicone oils. The antiblocking agent is generally applied to the beadsvia mixing, spray application, drum application, or other conventionalprocesses. It is usually used in amounts of from 0.01 to 20 parts byweight, preferably from 0.1 to 10 parts by weight, particularlypreferably from 0.5 to 6 parts by weight, based on 100 parts by weightof the TPU.

In all cases the product is expanded TPU beads. Preferred densities arefrom 5 to 600 g/l, and particularly preferably from 10 to 300 g/l.

The expanded beads are generally at least approximately spherical andtheir diameter is usually from 0.2 to 20 mm, preferably from 0.5 to 15mm, and in particular from 1 to 12 mm. In the case of non-spherical,e.g. elongate or cylindrical, beads, diameter means the longestdimension.

Foams can be produced from the inventive expanded TPU beads, for exampleby fusing them to one another in a closed mold with exposure to heat.For this, the beads are charged to the mold and, once the mold has beenclosed, steam or hot air is supplied, thus further expanding the beadsand fusing them to one another to give foam, whose density is preferablyin the range from 8 to 600 g/l. The foams can be semifinished products,for example sheets, profiles, or webs, or finished moldings with simpleor complicated geometry. The expression TPU foam therefore includessemifinished foam products and includes foam moldings.

The temperature during the fusion of the expanded TPU beads ispreferably from 100° C. to 140° C. The present invention therefore alsoprovides processes for production of foam based on thermoplasticpolyurethane, where the inventive expanded thermoplastic polyurethane isfused by means of steam at a temperature of from 100° C. to 140° C., togive a molding.

The invention also provides for the use of the expanded TPU beads forproduction of TPU foams, and provides TPU foams obtainable from theexpanded TPU beads.

The inventive foams can be recycled by a thermoplastic route withoutdifficulty. For this, the foamed TPUs are extruded, using a ventedextruder, and there can be mechanical comminution prior to thisextrusion process. They can then be processed again to give foams in themanner described above.

The inventive foams are preferably used in energy-absorbing moldings andin moldings for automobile interiors.

Particular preference is therefore also given to the following productscomprising the inventive foams: helmet shells, knee protectors, elbowprotectors, shoe soles, midsoles, insoles, and the following parts whichcomprise the inventive foams: steering wheel parts, door side parts, andfoot well parts.

The examples below are intended for further illustration of theinvention:

TABLE 1 Soft phase Thermoplastic polyurethane Composition [mol]Composition [mol] 1,4- Molar 1,4- Shore Adipic Butane- Poly- mass SoftButane- 4,4′- hard- TPU acid diol THF [g/mol] phase diol MDI ness A 1 1— 800 1.00 0.44 1.44 A78 B — — 1 1333 1.00 0.97 1.97 A72

The Shore hardness of the PU elastomers was determined to DIN 53 505.

EXAMPLE 1 Foam Bead Production

100 parts of the TPUs stated in Table 1 in the form of pellets eachweighing about 2 mg, 250 parts by weight of water, 6.7 parts oftricalcium phosphate, and 20 parts of n-butane were introduced, withstirring, into an autoclave and heated to the temperature stated inTable 2. The contents of the pressure vessel were then dischargedthrough a basal valve and depressurized, while the pressure in the tankwas kept constant by introducing, under pressure, nitrogen or theblowing agent used. The foam beads were freed from adherent residues ofauxiliaries via washing with nitric acid and water and were air-dried at50° C.

The impregnation conditions and the resultant bulk densities of theexpanded beads are found in Table 2.

TABLE 2 TPU of n- Butane Temperature Bulk density Table 1 [parts byweight] [° C.] [g/L] A 20 112 300 A 20 114 170 B 20 119 240 B 20 120 190B 20 122 140 B 20 125 120

EXAMPLE 2 Production of Moldings

The foam beads produced in Example 1 were charged into a preheated mold,with pressure and compaction. The mold was heated by steam at from 1.0to 4.0 bar, i.e. at temperatures of from 100° C. to 140° C., onalternate sides.

The mold was then depressurized and cooled with water and, respectively,air, and opened, and the mechanically stable molding was removed.

1. The process according to claim 12, wherein the Shore hardness of thethermoplastic polyurethane is from A 62 to A
 80. 2. The processaccording to claim 12, wherein the thermoplastic polyurethane exhibits amelting range, measured by DSC using a heating rate of 20 K/min, thatstarts below 130° C., and the thermoplastic polyurethane has at most amelt flow rate of 250 g/10 min at 190° C. with an applied weight of 21.6kg to DIN EN ISO
 1133. 3. The process according to claim 12, wherein thethermoplastic polyurethane comprises a polytetrahydrofuran of a molarmass of from 600 g/mol to 2500 g/mol.
 4. The process according to claim12, wherein the thermoplastic polyurethane comprises a polyester alcoholof a molar mass of from 600 g/mol to 900 g/mol.
 5. (canceled)
 6. Theprocess according to claim 12, wherein the thermoplastic polyurethanecomprising blowing agent comprises from 5 to 80% by weight of organicand/or inorganic fillers, based on the total weight of the thermoplasticpolyurethane comprising blowing agent. 7-11. (canceled)
 12. A processfor production of expanded thermoplastic polyurethane foam beads,comprising: (i) melting a thermoplastic polyurethane whose Shorehardness is from A 44 to A 84, together with from 0.1 to 40% by weight,based on the total weight of pellets, of a blowing agent, if appropriatewith additives, in an extruder, and (ii) pelletizing the melt withoutdevices which inhibit foaming to produce expanded thermoplasticpolyurethane foam beads. 13-14. (canceled)
 15. A process for productionof foam moldings, which comprises producing expanded thermoplasticpolyurethane foam beads according to the process of claim 12 and fusingthem in a closed mold by steam at a temperature of from 100° C. to 140°C.
 16. The process according to claim 12, wherein the thermoplasticpolyurethane is impregnated in (i) with 1 to 30 parts by weight, basedon 100 parts by weight of the thermoplastic polyurethane, of a C₄₋₁₀alkane as blowing agent.
 17. The process according to claim 12 wherein adensity of the expanded thermoplastic polyurethane foam beads is from 10to 300 g/l.
 18. The process according to claim 12 wherein a diameter ofthe expanded thermoplastic polyurethane foam beads is from 1 to 12 mm.